The present invention relates to a magnetoresistive sensor element, in particular an angle sensor element, as generically defined by the preamble to claim 1, and to a method for determining a direction of a magnetic field, as generically defined by the preamble to claim 6.
Sensors, in particular angle sensors, that operate on the basis of the magnetoresistive effect are known. In them, the electrical resistance of sensor elements is measured as a function of the direction of an external magnetic field. Especially the so-called AMR sensors, which exploit the anisotropic magnetoresistive effect, are used. Systems have also been described in which so-called GMR sensor elements (for giant magneto-resistance effect), especially using self-stabilizing magnetic layers, have been used (van den Berg et al, GMR angle detector with an artificial antiferromagnetic subsystem, Journal of Magnetism and Magnetic Materials 165 (1997) 524-528). Here a first thin, so-called reference direction is created by placing an antiferromagnetic coupling layer (of Cu or Ru, for instance) is placed between two oppositely magnetized layers (for instance of Co). The magnetic stability of the reference layer is enhanced by approximately one order of magnitude by this multilayered structure compared with individual Co layers. The direction of magnetization of the reference layer, the so-called reference direction, does not (in an ideal case) depend on the direction of the external magnetic field (the magnetic field to be measured).
The reference layer is covered with a thin nonmagnetic layer, over which in turn a thin soft magnetic layer, the so-called detection layer, is embodied. The detection layer orients its magnetization in the direction of an external magnetic field. It is known from the theory of the magnetoresistive effect that a sensor signal obeys a function R(xcex1)=R0+xcex94R*sin(xcex1), or R(xcex1)=R0+xcex94R*cos(xcex1), in which R0 is an offset resistance, xcex94R is a signal rise of the sensor, and a is the angle to be measured between a selected sensor direction (in particular the reference direction) and the direction of the external magnetic field.
Such AMR or GMR sensors can be used as 360xc2x0 angle sensors only with major effort. In particular, achieving sufficiently precise measurement results requires interconnecting at least two sensor elements, whose respective sensor signals have to be linked by computer. When GMR materials are used, a destruction of the sensor function if the magnetic fields are too strong is also demonstrated.
Angle sensor elements operating on a Hall basis are also known, but typically they can cover an angular range of only 120xc2x0.
The object of the invention is therefore to create a sensor, in particular an angle sensor, with which the effort and expense for furnishing it and for measurement during operation can be reduced compared with conventional sensors.
This object is attained by a sensor element having the characteristics of claim 1 and by a method for determining the direction of a magnetic field having the characteristics of claim 6.
According to the invention, a sensor element or a sensor is now created that compared with conventional devices of this type has a substantially simpler structure that can be attained more economically. It is no longer necessary to interconnect a number of sensor elements; an angle to be measured can be ascertained in a simple way with only a single sensor element. Thus there is no need for expensively mounting a plurality of sensor elements on a substrate. The offset and sensitivity of the sensor element are improved, since there is no need to calibrate different sensor elements.
Preferred fields of use for the sensor element of the invention are steering wheel angle transducers for regulating the dynamics of motor vehicle operation, camshaft signal transducers, for example for controlling direct starting of an engine, throttle adjusting units, or sliding roof regulators.
Advantageous features of the sensor element and the method of the invention are the subject of the dependent claims.
It is especially preferred that the different reference directions be offset from one another by 90xc2x0. With this provision, linearly independent signals, in particular signals associated with the sine and cosine of the direction of rotation of the external magnetic field, can for instance be generated in a simple way. By using the arc tangent (arctan) function, it is then possible in a known manner at little expense to determine the direction of rotation or the angle of the external magnetic field with respect to a select direction, such as one of the two reference directions.
Expendiently, the means are formed by a current conductor, which is galvanically separated from the first layer by means for selective orientation of the direction of magnetization of an insulator layer and is embodied to carry current in different directions, in particular in directions offset by 90xc2x0 from one another. With a so-called bias current of this kind, the selective orientation of the direction of magnetization can be attained in a simple and reliable way. Particularly by regulating the current intensity of the bias current, it is possible to adapt the sensor precision to the magnetic environment. No thermal drift in the sensor element occurs, since the bias current is constant over time or can easily be regulated to be constant. Since no hard magnetic material has to be used to create the reference magnetization, according to the invention in the event of strong magnetic fields there is no impairment or destruction of the sensor function. Such sensor elements have a wide temperature range in which they can be used and are usable in particular for motor vehicles.
Expediently, the first layer is made from a soft magnetic material. Such materials can be procured economically and can be magnetized by means of a current conductor-induced magnetic field (bias current).
Advantageously, the third layer, the detection layer of the sensor element, is also made from a soft magnetic material. This makes a precise, delay-free adaptation of the direction of magnetization of the detection layer to the direction of the external magnetic field attainable.
In a preferred embodiment of the method of the invention, one of the sensor signals is a signal associated with the sine of the angle between the first reference direction and the direction of magnetization of the third layer, and a further one of the sensor signals is a signal associated with the cosine of the angle between the first reference direction and the direction of magnetization of the third layer. Such signals can be evaluated in a simple way, especially using the arctan function.